Stereotactic surgery

Stereotactic techniques developed initially for lesion making, enable accurate placement of a cannula or electrode at a predetermined target site within the brain with the least risk.

Many different stereotactic frames have been developed, e.g. Leksell, Todd-Wells, Guiot. These, combined with radiological landmarks (usually ventriculography) and a brain atlas, provide anatomical localisation to within ± 1 mm. Since some functional variability occurs at each anatomical site, electrode localisation is also based on the recorded neuronal activity and on the effects of electrical stimulation.

CT/MRI STEREOTACTIC SYSTEM

CT and MRI compatible stereotactic systems allow cannula insertion to any point selected on the image. They are all based on the concept of identifiable external reference (fiducial) markers, e.g. Brown-Robert-Wells (BRW) system:

The position of the localising rods on the CT image permits calculation of the co-ordinates of a selected target

The position of the localising rods on the CT image permits calculation of the co-ordinates of a selected target

Biopsy

A head ring is \ /"®iii\0 forceps attached to the i skull and locating II After confirming the probe position on rods are inserted a target simulator, the localising rods are removed and replaced with an arc guidance system. This allows insertion of the probe or biopsy forceps, to the target position from any desired direction.

CT/MRI stereosurgery provides the optimal method for the biopsy or aspiration of small, deeply situated tumours or abscesses. Many now use stereotactic biopsy, for larger tumours. It carries lower risk than handheld biopsy and allows selection of specific areas within the tumour. Functional stereotaxy e.g. thalamotomy, pallidotomy, still requires electrical stimulation for the final localisation.

When combined with craniotomy it permits direct macroscopic examination of a lesion and may aid localisation, e.g. a small arteriovenous malformation. The improved resolution now available with CT/MRI scanning has led to sufficient anatomical localisation for accurate lesion making, obviating the need for ventriculography.

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